During the assembly of electronic circuits, such as integrated circuit chips, lead wires are bonded to contact pads on a chip in an automated operation that uses a bonding tool called a thermode. The leads may be in a machine-fed tape, and the bonding tool, heated with a heating element, is automatically pressed on the leads for a predetermined time to bond the leads to the pads. The temperature of the bonding tool tip, and the force it applies, results in the desired bond. The tool is then lifted, the next set of parts is moved into position, and the operation is repeated.
The bonding operation is preferably performed as quickly as possible to prevent unwanted heating of the chip, to minimize sticking between the tool tip and the leads, and to increase production speed. The tip of the bonding tool must be brought to an elevated temperature, and as it loses heat during each bonding cycle it must quickly be returned to the desired temperature for the next cycle. The bonding tool tip is also subjected to large compressive forces, so the structure thereof must be able to withstand such forces, without deforming or breaking, for many thousands of operating cycles. Materials such as titanium carbide and cubic boron nitride have been commonly employed as the tips of bonding tools.
The properties needed for a good bonding tool include excellent heat conductivity, strength, stiffness, and low coefficient of thermal expansion. Diamond, possessing these properties, has accordingly been used as the tip of lead bonding tools. In one prior art bonding tool design, a natural diamond is mounted in a metal matrix, such as a tungsten powder, that is held in an Inconel shank. A metal binder, such as silver or copper alloy, covers the diamond. The assembly is heated in a furnace to melt the binder, which infiltrates the matrix. Upon cooling, the binder solidifies, consolidates the matrix, and secures the diamond in place. The diamond is then machined, such as by grinding, into a desired tip shape. In another prior art approach, a diamond is pre-shaped into a desired tip configuration and then brazed to an Inconel shank which may optionally have an insert, such as of molybdenum. The cost of natural diamond, and the difficulty of securing it and forming it into a desired shape, are clear disadvantages of these types of bonding tools.
Kerschner et al., IEEE Transactions On Components, Hybrids, and Manufacturing Technology, Vol. CHMT-2 NO. 3, 1979, disclose a thermode having a diamond tip attached to an Inconel body. The diamond tip contains a laser machined cavity to accommodate devices to be bonded.
EPA Publication No. 032,437 discloses a thermocompression bonding tool having a tip formed of a mass of synthetic polycrystalline diamond material sintered in a predetermined form and mounted on a cemented tungsten carbide substrate.
U.S. Pat. No. 4,932,582 discloses a method for preparation of a bonding tool. The superhard material of the tool can be single crystal diamond, diamond compacts, CBN compacts, cemented carbides, molybdenum and the like, and unified bodies of mixtures thereof. Reference is also made in this patent to prior art bonding tools using sintered metal powders holding a single crystal diamond head, and brazing of single crystal diamond to tungsten or molybdenum shanks.
U.S. Pat. No. 4,943,488 discloses a thermode which includes a "TSPCD" (temperature stable polycrystalline diamond) element bonded to a support or to an insert to be received in a support.
Although the described approaches have advanced the thermode art, there is substantial room for improvement in one or more of the following areas:
The use of sintered diamond compacts as a tip can result in thermally unstable structures and/or the presence of impurities which can cause sticking or other operational problems.
The forming of extremely hard thermode tips into the variety of shapes needed for different bonding jobs tends to be difficult and/or expensive.
Adherence of the thermode tip to a holder or to a substrate, and/or adherence of the substrate to a holder, may be inadequate for long term use.
The thermal conductivity, stiffness, and/or coefficient of expansion of the tip, substrate, and/or other portion of the holder may limit the efficiency of the bonding tool.
It is among the objects of the present invention to address these and other limitations of the prior art in the fabrication and structure of thermocompression bonding tools.